1. Field of the Invention
This invention relates to a process for annealing thin films, wherein the annealing is performed in a ceramic case (xe2x80x9cbombxe2x80x9d or chamber) having the same chemical composition as the particular film itself.
2. Description of the Previously Published Art
Knauss et al., xe2x80x9cThe Effect of Annealing on the Structure and Dielectric Properties of Bax Sr(1-X)TiO3 (BST) Ferroelectric Thin Films,xe2x80x9d Appl. Phys. Lett., vol. 69(1), pp. 25-27 (Jul. 1, 1996) relates to post-deposition annealing of SBT performed at 900xc2x0 C. for eight hours. The entire contents of this article are incorporated herein by reference. There is no teaching as to the use of any enclosed annealing chamber.
Varadan et al., xe2x80x9cCeramic Phase Shifters for Electronically Steerable Antenna Systems,xe2x80x9d Microwave Journal, pp. 116-127 (January, 1992) relates to the use of the materials to develop a new class of tunable microwave devices. It describes the important issue of finding ways to reduce the dielectric loss. This article is only about bulk material, however, and it does not relate to thin films. It shows that ferroelectric phase shifters for electronically steerable antenna systems can be realized by using an appropriate composition (e.g., Ba0.45Sr0.55TiO3) of barium strontium titanate material in a suitable transmission line medium (waveguide, coaxial line, or microstrip line). Ferroelectric phase shifters will have several advantages over other types of phase shifters, including high power handling capacity, simple driver circuitry, low drive power, and low cost, because of the large variation (more than 50%) of dielectric constant with DC biasing voltage.
Horwitz et al., xe2x80x9cBaxSr(1-x)TiO3 Thin Films for Active Microwave Device Applications,xe2x80x9d Integrated Ferroelectrics, vol. 8, 53 (1995) relates to the variation in thin film properties as a function of composition (i.e., ratio of Ba/Sr). There is no discussion about annealing. The dielectric constant, loss tangent and Curie temperature for Srx Ba(1-x)TiO3 (SBT) thin films with x=0.2-0.8 have been investigated at microwave frequencies. SBT films (0.5-3 xcexcm thick) were grown on (100) MgO and LaAlO3 substrates by pulsed laser deposition at substrate temperatures from 850-900xc2x0 C. in 0.35 Torr of oxygen. Deposited ferroelectric films were single phase, highly oriented, and characterized by X-ray rocking curve widths of xe2x89xa60.5 degrees. Highly oriented SBT films with X-ray rocking curve widths of 72 arc seconds were observed. In general, the thin film dielectric constant at microwave frequencies is low (200-950) compared to the reported bulk value, but strongly dependent on the Sr/Ba ratio. Biasing of a ferroelectric interdigital capacitor ( less than 200 kV/cm) produces a change in the dielectric constant which resulted in a phase shift in the reflected signal (S11) measured as a function of frequency from 100 MHz to 10 GHz. The dielectric loss tangent measurement, as measured at room temperature and 9.2 GHz, ranges from 0.1 to 1.2xc3x9710xe2x88x923 and depends on the Sr/Ba ratio. These data show that SBT thin films are suitable for the development of frequency tunable microwave circuits and components.
3. Objects of the Invention
It is an object of this invention to provide an improved method of annealing thin films on a substrate so that the properties of the film are closer to those of a single crystal of that thin film material.
It is a further object of this invention to provide a method of annealing a multicomponent film on a substrate such that during the annealing the multicomponent film does not loose any of its components.
It is a further object of this invention to provide a method of annealing a BaxSr(1-x)TiO3 (BST) film on a substrate at temperatures greater than 900xc2x0 C.
It is a further object of this invention to provide a method of annealing a multicomponent film on a substrate in which the film is annealed in a xe2x80x9cbombxe2x80x9d or chamber which is made of a material having at least all of the relatively volatile components of multicomponent material of the film.
These and further objects of the invention will become apparent as the description of the invention proceeds.
An improved method has been developed for annealing a multicomponent film on a substrate. The substrate with the multicomponent film thereon is enclosed within a vessel. The multicomponent film is formed of a first multicomponent material and the vessel is formed of a second multicomponent material which has at least all of the components of the first multicomponent material or in the case where there are nonvolatile components, then the vessel is formed of a second multicomponent material which has at least the same composition of relatively volatile components as the first multicomponent film. The multicomponent film is annealed within the vessel for a time sufficient to anneal the multicomponent film. During this annealing the multicomponent film remains in contact with a vapor of the first multicomponent material and the second multicomponent material and this resulting vapor prevents the multicomponent film from losing components of its first multicomponent composition during the annealing. This process prevents any compositional change in the film and it provides for improved dielectric behavior of the thin film. Preferred thin film materials are ferroelectric materials. However, the invention need not be limited to a method of treating ferroelectric thin films. The invention, in its broadest sense, relates to a method of treating a thin film of any material on a substrate by bomb annealing.